An embodiment of the invention relates to a method and apparatus for correction of an image and in particular, the automatic estimating a correction to be made to a control image, deformed with respect to contrast images. One application field of the embodiment is medical imaging and it relates particularly to digital subtraction angiography (DSA).
Digital subtraction angiography is an image acquisition method using for example, a radiography apparatus emitting in particular X-rays, in which an attempt is made to represent the vascularization structures of the tissues of an object such as a patient. In angiography, vascular structures are demonstrated by comparing two images acquired with and without the injection of a contrast product into the patient's network of blood vessels. Point by point subtraction of the first control image called the “mask” obtained before the contrast product is injected, from the image obtained with the injected contrast product provides a means of obtaining an image of vascular structures alone.
A problem with the above method is due to the patient movement between one acquired image and the next. This movement may apply to all or some of the patient's body. The result is artifacts in the subtraction image that can modify the representation of vessels injected with the contrast product or mean that structures not injected with the contrast product are interpreted as blood vessels.
Thus, these artifacts not only degrade the quality of the subtraction image, but they may also mislead the radiologist with regard to the second image with the contrast product. In an attempt to eliminate subtraction artifacts, the radiologist can manually shift the mask image with respect to images taken with the contrast product, so as to make them correspond as much as possible before subtraction. However, manual shifting of the mask is usually done by trial and error and is a long interactive process that does not always produce an optimum result.
U.S. Pat. No. 5,048,103 describes a method of automatically resetting a mask image with respect to another contrast image in which a set of landmarks are selected and their shift is estimated between images so as to restore the mask image, the landmarks being selected as a function of how a combination of five magnitudes X1, X2, X3, X4 and X5 can be satisfied, the combination having five coefficients B0, B1, B2, B3, B4 and B5 determined by a statistical analytical assessment on a population of images of the same type, for which the characteristics of restoration are already known. This method has the disadvantage not only of being complicated to use, but it also takes up a great deal of computer calculation time.